The present invention relates to apparatus for casting of low-density alloys and especially alloys such as magnesium-based alloys which require protection from oxidation during the pouring operation. In particular, the invention relates to apparatus for the casting of such metals into ceramic or sand molds in which the castings are thin walled and consequently difficult to cast because of problems with incomplete filling of the molds.
There has always been a problem in casting magnesium-based alloys in thin section because of the low density of the magnesium which adversely affects its fluidity. Fluidity, in a practical sense, depends upon the heat content of the molten metal, which is made up of the specific heat of the liquid and the heat of fusion which is given up during solidification. On a bulk or volumetric basis, the heat contained in magnesium at the time of pouring is relatively low. Therefore, the heat lost in the flow of the metal through the mold passages rapidly reduces the available heat to the point that solidification and flow stoppage tend to occur before the mold cavity is completely filled.
To overcome this danger of non-fill by premature solidification, the fluidity can be improved by increasing the metal velocity. The velocity is maximized by maintaining the columnar height of the liquid above the mold cavity as high as possible since the metallostatic pressure controls the velocity. But the low density of magnesium is a hindrance because the metallostatic pressure of a liquid metal is a function of the metal density.
In the lost wax molds for magnesium, the velocity can be increased by placing the plaster-binder solid mold over a vacuum port. The port tends to evacuate the air in the interstices of the mold thus allowing the ambient air pressure to accelerate the entry of the metal into the mold. Although this commonly used method is better than relying solely on gravity, it is only marginally effective. The relative ineffectiveness is due to the fact that evacuation of air from the mold cavity cannot occur until the incoming liquid metal fills the ingates and thus seals the mold cavity from the ambient atmosphere. When incoming liquid effects this seal, the evacuation of the cavity can commence. Evacuation cannot be rapid because the vacuum port must work against the resistance of the fine passageways in the refractory mold that comprise the interstices between the fine refractory particles. The velocity attained by the metal under these circumstances is limited even for small castings with small volumes to be evacuated, but, for large castings, the velocity effect is much worse and consequently the nonfill problem is even greater.
Another difficulty in pouring magnesium alloys is the constant need to prevent oxidation and burning of the liquid metal. This is commonly avoided by injecting sulfur dioxide gas into the mold and by dusting sulfur on the surface of the metal. The presence of this gas in the mold hinders fluidity because the gas must be displaced before the metal can completely fill the mold cavity.
Fluxes must also be used to prevent burning of the magnesium while it is being melted. Before pouring, the flux must be skimmed off the surface of the metal to prevent its entrapment in the casting. To reduce the danger of flux inclusions being incorporated in the stream of metal from the pouring ladle, modified ladles in teapot form are often used. This expedient reduces the flux problem but by no means eliminates it.
One casting technique which is capable of overcoming some of these problems is disclosed in a paper entitled "Method of Casting with Counterpressure", by Balevski and Dimov, dated Nov. 24, 1971, and delivered at the Bulgarian Science and Technology Days in London. The technique involves producing a "counterpressure" in the mold and then displacing the metal into the mold by the action of another greater pressure, which can be produced pneumatically, by a piston or by gravity, i.e., the metallostatic pressure of a column of metal. But the apparatus disclosed for effecting this technique is cumbersome and inefficient. The pneumatic means and piston-actuated means operate to drive the molten metal from the bottom of a vessel upwardly into an overlying mold cavity. Thus, the system has to overcome the gravitational forces on the molten metal. The gravity-operated apparatus disclosed involves a movable assembly which involves inversion of the mold and the feeding reservoir, which would be extremely costly and difficult to construct.